Tracking 1037 people born in Dunedin in 1972/3, they interviewed participants at 18, 21, 26, 32 and 38 years and recorded whether they were dependent on cannabis at the time.

People who were cannabis dependent at 18 had lower IQs at age 38 and the IQ fall was most severe if they remained dependent into early adulthood, dropping by an average of 8 IQ points.

For a person starting with average intelligence (IQ=100), an 8 point fall would put them in the bottom 30 per cent of the population for IQ, the authors say.

Adult use may be less damaging

As expected when tracking a cohort of the general population, only a small number (52) became cannabis dependent in their teens, but they showed deficits affecting not only IQ, but memory, attention and speed of processing, the authors say.

Importantly, a further 92 people became cannabis dependent as adults, but they did not show a significant IQ drop, suggesting that the detrimental effect is specific to cannabis during adolescence.

The authors were able to rule out alcohol, hard drugs, recent cannabis use and tobacco as being responsible for the IQ decline.

And although cannabis users tend to drop out of school, lack of education was also excluded as a cause.

Adolescent brain 'vulnerable'

"We know that the brain is undergoing important critical developmental changes from adolescence through the early 20s," says Meier, who suspects that cannabis is damaging at this crucial time.

"I think the study shows that cannabis use in adolescence can have long-term effects on mental abilities," she says, advising adolescents not to use it because their brains are "vulnerable and still developing".

If people are determined to use it, we should be concentrating on delaying use until adulthood, she argues.

'Well-conducted' study

"You don't get any better information than this," he says, adding that it is a large and well- conducted study.

"It adds to a growing body of knowledge that cannabis is not an entirely benign compound," remarks Large.

He says we need to be informed about the potential consequences of its use, particularly as it is popular amongst adolescents. "Twelve year olds in Australia are more likely to have smoked a joint than a cigarette," he says.